A lot of women have stomach cramps with their periods. It is not unusual. This is caused by the muscular contractions of the uterus whilst expelling the blood - shedding its lining, so that it can rebuild for your next cycle. As the cramps are contractions, it is not unsimilar to the kind of pains that you have in labour, although not as intense.

Try putting a heat pad on your belly and this should help with the pain a lot. Heat helps to relax all the muscles, thereby lessening the pain.